There are technical applications in the microwave field where the problem may arise of making a connection between two waveguides which are movable relative to one another. In particular this problem may arise in the case where a telescopic antenna is to be used for instance in a shelter or, in a submarine, where a radar antenna may be fixed to the top of the slidable part of its retractable periscope mast. When the craft is submerged, the mast of the periscope is retracted, whereas during navigation on the surface the mast is extended so that the antenna is situated above metallic masses such as the conning tower, which would form a mask. In such an application, the line supplying microwave power to the antenna cannot be permanently connected to the transmitter/receiver of the radar. In fact, in one prior-art construction where the transmitter/receiver of the radar is situated in the control room, when the antenna is in the raised position the transmitter/receiver is connected to the microwave line by a device known as a "duck-bill". The connection is broken when the antenna is in the lowered position. The actual connection is achieved by applying two choked flanges together under pressure, one flange being secured to the microwave line and the other flange being secured to the transmitter/receiver assembly of the radar.
The disadvantage of such an arrangement is fairly obvious. Care has to be taken that the connection is properly made when the antenna is raised and that the flanges are not offset from one another by even a small amount at the moment of contact. Another disadvantage inherent in this arrangement for the radar of a submarine becomes apparent when it is desired to rotate the antenna, which is situated at the top of the movable part of the periscope mast. It is the mast itself, which is of circular cross-section, which turns, taking the antenna with it during its rotation, thus making it necessary for a rotary joint to be fitted at the base of the mast. However, if the mast rotates at high speed this gives rise to a wake which can easily be detected. To alleviate this disadvantage it is necessary to modify the shape of the mast.